Center contact structure for coaxial cable conductors



, ET AL 3,453,376

July 1, 1969 ca. w. ZiEGLER, JR.

CENTER CONTACT STRUCTURE FOR COAXIAL CABLE CONDUCTORS Sheet 7 PRIOR ART Filed July 5. 1966 y 1969 a. w. ZIEGLER, JR., E L 3,453,376

' CENTER CbNIACT STRUCTURE FOR COAXIAL CABLE CONDUCTORS Filed July 5. 1966 Sheet 2 Of 2 I fi 36 7 l6 1 I 9 I United States Patent 3,453,376 CENTER CONTACT STRUCTURE FOR COAXIAL CABLE CONDUCTORS George William Ziegler, Jr., Carlisle, and Armand Samuel Apa, Camp Hill, Pa., assignors to AMP Incorporated, Harrisburg, Pa.

Filed July 5, 1966, Ser. No. 562,572 Int. Cl. H02g /02, 15/08 US. Cl. 174-75 4 Claims ABSTRACT OF THE DISCLOSURE Background of the invention This invention relates to improvements in center contact structures for coaxial cable connectors.

In US. Patent No. 3,245,027 there is disclosed a type of coaxial connector which employs a center contact structure which fits into the center conductor of a coaxial cable. The coaxial cable there referred to is of a type which has a center conductor comprised of either a solid rod of conductive material or a tube of conductive material. In accordance with the invention there described in a use wherein the cable center conductor is solid rod the rod is prepared for use by drilling and tapping the center conductor and then fitting the center contact structure therein. In applications wherein the center conductor of the cable is hollow the conductor is merely tapped to provide threading to receive the center contact structure.

While the device of U8. Patent No. 3,245,027 has been accepted as a substantial improvement in the art of connecting coaxial cable for high frequency applications, cer tain problems have arisen in practice due to differences in the many cables of different sizes being manufactured. One of these problems is that the center conductor of such cable is not fixed against rotation within the cable which makes it difiicult, particularly in small sizes, to drill and tap the constructive material. Another problem arises if either the drill or tap is slightly oil center to result in a distortion and bulging of the center conductor. A change in center conductor diameter as will be appreciated by reading US. Patent No. 3,245,027 is undesirable in that it changes the characteristic impedance along an appreciable length of the transmission line formed by the connector. This change in characteristic impedance produces reflections and increases the losses of the connector as well as causing a related degradation of signal transfer. A second problem with respect to those coaxial cables having a thin wall hollow center conductor is that the tapping procedure even when on center may cause bulging. The cable usually has a relatively thin Wall section (sometimes on the order of thousandths of an inch) and is made to a tolerance of approximately 4 to 5 thousandths of an inch. A slight variation in tolerance of the cable center conductor material will result in the tap forming the threads bulging the outer diameter of the center conductor to result in an appreciable discontinuity.

A further and practical problem related to the use of the device in 3,245,027 and to similar devices is one of 3,453,376 Patented July 1, 1969 performance and cost of application. If the user takes a great deal of time to accurately position the drill and tap and to carefully avoid bulging, the cost of installation is vastly increased. Even with such care there are occasions when there will be no tap available which can be used with an undersized cable center conductor. As those skilled in the art will appreciate it is generally unacceptable to make the performance of a high quality design device overly dependent upon the skill of the user. What happens all too frequently is that the user in his eiTort to reduce installation time and overall cost speeds up installation procedures with the result that a device of superior design gets put together in a manner which results in poor performance.

Summary of the invention This invention relates to improvements in center contact structures for coaxial cable connectors. Accordingly, it is an object of this invention to provide a center contact structure for coaxial cable connectors which is arranged to facilitate an easy and reliable installation.

It is a further object of the invention to provide an improved center contact structure for coaxial cable connectors which eliminates the need for time consuming steps of cable preparation during installation of a connector onto a cable.

It is a still further object of the invention to provide a self-tapping center contact member for use with coaxial cable having a center conductor which is or has been hollowed out to receive a center contact structure inserted therein.

It is another object of the invention to provide a center contact member for coaxial cable connectors which has a design which better assures proper installation of contact structures and therefore improves the performance of coaxial devices used therewith.

The foregoing objectives are attained by providing a center contact member which is axially insertable into a hollow center conductor and includes cutting edges operating to bite into the material thereof to create a holding surface within the center conductor. The member includes a shank having spring characteristics to drive the cutting edges radially outward and a diameter to limit the penetration of the cutting edges to prevent bulging. The cutting edges of the member are supported by surfaces adjacent thereto which engage the holding surfaces caused by rotation of the cutting edges to resist axial pullout. The engagement effected thereby is in part against freshly exposed conductive material and is made to define a stable and low resistance interference.

In the drawings:

FIGURE 1 is a perspective of a coaxial connector attached to cable with portions shown in section to show the placement of the invention;

FIGURES 2-4 are perspectives showing the application of the invention center contact structure in one embodiment;

FIGURES 5-7 are sectional views showing installation procedure for the embodiment of FIGURE 2-A;

FIGURE 8 is a sectional view of an alternative installation procedure;

FIGURE 9 is an end view of an alternative embodiment of the center contact structure shown in FIGURE 4-7;

FIGURES 10 and 11 are sectional views of an alternative contact structure; and

FIGURE 12 is a sectional view showing the problem faced by the prior art.

Description of the preferred embodiment In FIGURE 1 there is shown a coaxial connector arranged to electrically and mehcanically interconnect separate coaxial cables shown as 10. The connector of FIG 3 URE l is comprised of halves 18 and 24 which each include portions to provide mating of the halves in a manner to serve as a transmission path for signal energy carried over the cables 10. US. Patent No. 3,245,027 is referred to for details as to the arrangement and detail .features of the connector shown in FIGURE 1. It is contemplated that a wide variety of different coaxial connector designs may be served by the invention and the connector shown in FIGURE 1 is included to generally teach the environment of use of the invention.

For the purposes of disclosure the important points to consider relative to the connector are the characteristics of the cable which is comprised of an outer conductive tubular member 12 surrounding a sheath of dielectric material 14 and a center conductor 16. The center conductor 16 as shown in FIGURE 1 is formed of a solid copper rod which has been drilled and tapped as indicated in FIGURE 1 to receive the threaded end of a center contact structure. The structure is shown as 20 which is in the form of a female receptacle adapted to receive a male contact member shown as 26 in the half 24. The male member 26 is connected to the center conductor of the cable to the left in the same manner as that shown with respect to half 18. Various types of cable are presently being employed and is contemplated that the invention may be used with such types with equal facility as with the exact type shown. For example, in certain types of cable the dielectric material shown as 14 is formed of space dielectric discs which support the center conductor 16. Other types include a dielectric formed of a spirally disposed bead of dielectric material which extends helically along the cable to support the center conductor coaxially inside the outer conductor 12. Still other types include a center conductor which is formed of a hollow copper tubing. Typically, this tubing has a thin wall section and it is such thinness of wall section which has caused a problem in prior devices.-

Referring to this problem with respect to solid conductive'material, FIGURE 12 shows how the center conductor has been prepared in accordance with prior techniques. First the center conductor is drilled to a depth as indicated in FIGURE 12 and then a tap is inserted to provide threading of the interior surface of the conductor. In the case of hollow center conductors the drilling step is not necessary and the tapping proceudre is performed directly on the center conductor. In either event the problem has been that the tap tends to cause a bulging of the conductive material as indicated in FIGURE 12 along the length of the hole which receives the center contact members 20 or 26. This bulging is of a substantial length and it creates a change in diameter of the inner conductive path of the connector or cable along the length thereof which discontinuity causes signal reflections, degradation, low VSWR, and generally poor performance of the connector. As will be appreciated from a reading of 3,245,027 the particular connector design shown in FIGURE 1 is for high performance applications wherein it is important that the connector pass signal frequencies over a broad range extending up to 10 gHz. and beyond with a minimum VSWR.

The invention in one embodiment contemplates a center contact structure as indicated in FIGURE 2 by the numeral 30 and as indicated in FIGURE 4 by the numeral 50. The forward ends of 30 and 50 are the mating center conductor halves for two connector halves and the rearward ends thereof are contemplated as being identical. In FIGURE 2 a cable is shown to have a hollowed out center conductor 16. This may be assumed to be either a drilled solid conductor or as is frequently the case a hollow tubular member which does not require drilling. The member 30 and as previously mentioned the member 50 each include a portion which extends inside the bore in 16. This portion relative to 30 includes a shank 34 slotted as at 36 along its length to provide a bifurcation and give the shank spring characteristics. Between the large end of 30 and shank 34 is a very slight chamfer 33. The end of 34 is made to include a portion 38 which is beveled inwardly to facilitate insertion of 30 Within the bore in 16 and includes a sharply radiused corner shown as 39 in FIGURE 2 at the point of maximum diameter of 38. The remainder of the shank 34 is of a diameter reduced relative to the diameter at 39. The diameter of 34 is controlled so that at its maximum (including tolerance) it will fit easily within the minimum (including tolerance) diameter of the bore of 16 shown as d. The shank 34 is made to be of a length to extend well dovm within 16 to establish a substantial spring force operating on 39 without being overly stiff.

Details of 30 and the installation thereof are better shown in FIGURES 5-7. In FIGURE 5, 30 is shown positioned prior to insertion within 16. As can be seen the diameter of 39 is slightly larger than that of the diameter d within 16 but the bevel permits an easy insertion of 30. In FIGURE 6 a member 30 is shown inserted within 16. It will be noted that the corner at 39 loads the shank spring arms 34 biasing them inwardly. The member 30 is preferably made of a relatively hard conductive material and while the spring action defined by the bifurcation of 34 is substantial the member can be inserted by hand force. With the contact structure 30 positioned as shown at FIGURE 6 it is then rotated as indicated in FIGURE 7. It has been found that one or two turns are sufficient to cause the sharp leading edges (at the intersection of slot 36 and corner 39) to dig into the conductive material of 16 until the diameter of 34 is reached and the surface of 34 is in substantial contact with the inner surface of 16 at least at the end near 39. It has been found that the torsional loading applied after insertion causes the edges to operate to bite into the material of 16 with the remainder of the surface area of the corner of 39 following the edges down into the groove created in 16. Control of the diameter of 34 absolutely precludes bulging of the outer surface of the center conductor. As 30 is rotated in the manner shown in FIGURE 7 any chips or particles cut from the material of 16 are accommodated by the slot shown as 36. The chamfer 33 serves to center the member 30 in 16 in firm engagement therewith to prevent looseness in use with the larger tolerance conductors without causing a bulge.

In an actual unit members like 30 and 50 were dimensioned (in inches) as follows. Each member had a maximum diameter of about approximately 0.248 for use with a cable having a center conductor having a diameter of approximately 0.248. The center conductor had an inner bore of a little more than 0.202 and the diameter of the shank 34 was made to be about 0.202. The diameter at 39 was made to be about 0.212 to thus limit the penetration thereof into the wall material of the outer conductor to about 0.005. The overall length of the shank was made to be about 0.750 with the portion 38 being made of a length approximately 0.120. The edges at 39 were left as machined and were not de-burred. In use the unit was inserted within a cable and given two full turns. The pull-out force resulting exceeded 15 pounds. In the typical application, as in the connector previously described, the center contact structure is not utilized to mechanically hold the cables together and 5 to 12 ounces of pull-out force is generally sufficient. The unit was comprised of beryllium copper machined to the configuration shown and silver plated. An electrical continuity check showed that a low resistance resulted.

In FIGURE 8 an alternative embodiment of the invention is shown wherein the center conductor is reamed at one place shown as 41 prior to insertion of the member 30. This may be accomplished by a number of procedures including the use of a bit shaped like 30 which is compressible for withdrawal. The extra axial space provided at 41 operates to assure an easy fit of the member 30. The member 30 is then snapped into place. This approach may be preferred if the center conductive material is particularly hard relative to the conductive material of the member 30. For example, if the member 30 must for other reasons be of brass and the center conductor is of a hardened material such as stainless steel the method and technique indicated by FIGURE 8 may be preferred.

FIGURE 9 shows an alternative embodiment wherein the bifurcation formed in the shank 34 is given a slight axial offset exposing the corners 40 and 42 in the manner indicated but somewhat exaggerated in FIGURE 9. These corners then are better placed in a position to cut the material within the center conductive member. In an actual unit the offset was made to be between 0.003 and 0.005 of an inch. This increased the pull-out force to 65 pounds.

FIGURE 10 shows an alternative embodiment of the invention wherein a center contact structure 60 carrying a contact portion such as 62 is made to have a shank 64 carrying on the outside a series of threads 66. These threads are made to taper down to a smooth portion toward the forward end of the member which carries a portion such as 70 to facilitate insertion within the conductor. There is a slot provided as at 68 which permits the member to have spring characteristics in the forward end to ease insertion and to facilitate the selftapping technique which results from use of the unit. The slot also accommodates any particles cut from the inside of the center conductor 16. The depth of the threads 66 is held and limited carefully to avoid the bulging effect heretofore discussed. For example, in an actual unit for use with a center conductor having an outer diameter of approximately 0.248 of an inch and a bore of approximately 0.208 of an inch the threads are made to have a maximum depth of 0.008 of an inch; less than half the wall thickness of the center conductor. This was found not to bulge the center of the conductor of a cable in application and to provide a very substantial pull-out force relative to that actually required.

This latter embodiment was also found to provide even better electrical characteristics and may be preferred in applications wherein the center conductor is of hollow tubing which could conceivably have some greater amount of oxidation products on its inner surface than would be the case if solid tubing were freshly drilled. The latter embodiment is useful in applications wherein it is desirable to non-destructively remove the center contact structure.

With respect to the embodiments of FIGURES 2-7 removal may be provided by giving the face of 39 opposite to the bevel a slight chamfer which serves to cam the spring arms in upon axial withdrawal. Control of the angle of this chamfer can be readily made to provide a pull-out force exceeding 5 pounds but non-destructive to the conductor 16 and the center contact member.

With respect to all of the various embodiments it is contemplated that the shank may be split to provide more than the two arms shown. For example, the shank 34 could be made to have two slots 36 intersecting to provide four arms. The end structures 38 and 39 would be identical with additional biting edges provided at each slot.

In reference to the cutting or biting edges heretofore described it is important that sharpness be maintained in manufacture. In both embodiments the corner radius should be preferably no more than 0.003. In the embodiment wherein there is a spiral thread the flat or top of the thread should particularly be held to a minimum because it is the flat that causes bulging by radially pushing them aterial of the center conductor outwardly. It is also important to provide a sufiicient pitch in the threaded version to hold the force required to rotate the contact member into the center conductor to prevent swisting of the center conductor in those cables having a non-solid dielectric. For example, on a center conductor having a bore of approximately 0.208 of an inch the pitch might be on the order of 32 to 44 threads per inch.

Having now disclosed and described my invention in terms intended to enable its preferred practice, I define it through the appended claims.

What is claimed is:

1. As an article of manufacture for use with coaxial cable an improved contact structure including a forward portion having a contact surface thereon for mating connection with a further conductive member and a rearward portion for connecting said structure to the center conductor of a coaxial cable having a bore therein, said rearward portion having a diameter to be inserted into said bore and including means responsive to insertion and to rotation of said structure within said bore to cut into the material of said conductor and anchor said structure to said conductor against axial pullout and electrically connect said structure to said conductor, the said means including at least one axially extending spring arm having at least one cutting edge biased outwardly by said arm along a radius of the rearward portion, said arm extending slightly off center with respect to said rearward portion in a direction normal to said radius, to place said cutting edge outwardly of the body of said rearward portion to better facilitate the cutting operation of the edge.

2. In a coaxial cable having a center conductor of a uniform diameter throughout its length including a center bore and a contact member therein, said contact member being of conductive material including a first portion for connection to a conductive member and a second portion fit within the cable center conductor bore, the second portion including a spring arm carrying on the outer surface thereof at least one raised cutting edge biased by said arm outwardly of the axis of the contact member, means on the said contact member to guide said second portion of said contact member into said bore, the said raised cutting edge cutting into the inside wall of said bore and means on said contact member limiting the depth of penetration of said edge to avoid bulging of said conductor, the said edge being biased by said arm into engagement with the inside wall of the bore holding said contact member in electrical and mechanical engagement with said center conductor.

3. The article of manufacture of claim 2 wherein said second portion is tapered along the length thereof facilitating insertion within said conductor with a minimum force of rotation required.

4. Thearticle of claim 2 wherein said cutting edge is formed in a single plane transverse to the length axis of said second portion.

References Cited UNITED STATES PATENTS 1,114,366 10/1914 Knaulf' 339-252 XR 3,199,061 8/1965 Johnson et al. 17475.2 XR 3,219,751 11/ 1965' Pfendler et al. 174-752 2,490,632 12/1949 Andersson et al. 2,763,845 9/ 1956 Laube.

FOREIGN PATENTS 829,755 1/ 1952 Germany.

DARRELL L. CLAY, Primary Examiner.

U.S. Cl. X.R. 339-100, 177, 252 

